Patent application WO-2011/107,669 describes the coupling of a class A analog amplifier, made up of a reference voltage generator and a class D digital amplifier constituting a power voltage generator, which is coupled to the output of the reference voltage generator by an inductance, with which it then forms a current source.
The combination of a class A amplifier and a class D amplifier is intended to create an amplifier with a very high performance and very high linearity.
In practice, several phenomena limit the total performance of such an amplifier, its ability to reproduce high frequencies, and its maximum achievable power. These phenomena are in particular the following.
The switching losses of the MOS transistors of the class D amplifier subject to PWM control are proportional to the switching frequency. For this reason, in practice, this frequency cannot significantly exceed 500 kHz for voltages greater than 100 volts. Due to the PWM control and the inductive aspect of the coupling impedance, a ripple current is formed in the coupling impedance.
The amplitude of the current ripple in the impedance of the class D amplifier is inversely proportional to the switching frequency and inversely proportional to the value of the inductance. However, to be capable of reproducing the top of the audio spectrum, the used inductance must have a low value so as not to limit the slew rate in the audio band. Unfortunately, this low inductance value leads to a ripple current amplitude at the class D switching frequency with a high value. Yet this high-frequency current ripple is completely absorbed and dissipated by the class A analog amplifier, which causes significant heat dissipation.
Under these conditions, the class A amplifier is highly stressed when reproducing high frequencies, in particular exceeding 10 kHz.
There are two known solutions to decrease or cancel the high-frequency ripple of the class D amplifiers.
A first solution consists of using a wave trap (RLC) tuned to the working frequency of the class D PWM control, this wave trap being placed at the output of the amplifier powering the load.
This solution has two drawbacks, namely cancelling out only the fundamental of the high-frequency current ripple and introducing a phase shift affecting the harmonics of the signal, which is very detrimental to the feedback control of the control loop.
The second solution consists of using a “ripple steering” technique described in the AES Society publications. This technical is based to two strong structural hypotheses, namely:                a mandatory bridge operation of two class D amplifiers, the load not being connected to the ground, and        an operation of the two class D amplifiers in phase opposition.        
These hypotheses are often unwanted in an amplifier.
The D-Premier amplifier is also known, marketed by the company Devialet, which uses, as coupling impedance, two inductors mounted in series and the midpoint of which is connected to the ground by a resistance. The purpose of this resistance is to absorb the very high-frequency energy (in a frequency band greater than 20 MHz) due to the switching of the MOS transistor bridges present in the class D amplifier and does not contribute to the current ripple.